1. Field of the Invention
This invention relates to a polarization control element for obtaining a linearly polarized laser beam.
This invention also relates to a solid-state laser system which has a cavity resonator provided with the polarization control element, and produces a linearly polarized laser beam.
2. Description of the Prior Art
Solid-state laser systems are widely known in which a solid-state laser crystal added with rare earth materials such as neodymium is pumped by a semiconductor laser, or the like, as disclosed, for example, in Japanese Unexamined Patent Publication No. 62(1987)-189783. In this type of a solid-state laser system, it is widely practiced to put crystal made of a non-linear optical material in the cavity resonator so that the wavelength of a solid-state laser beam can be converted into, for example, a second harmonic wave in order to obtain a laser beam having a shorter wavelength.
By the way, as with other types of laser systems, there is a need for the generation of a linearly polarized laser beam and a linearly polarized laser beam in a single longitudinal mode. However, when an optically isotropic crystal such as a YAG crystal is used as a solid-state laser medium, the crystal in its present form cannot produce a linear polarized laser beam. In such a case, various techniques have been hitherto employed: for example, a Brewster plate for polarization control purposes is incorporated into a cavity resonator in order to obtain a linearly polarized laser beam; or a Brewster plate and an etalon are incorporated in a cavity resonator in order to obtain a linearly polarized laser beam in a single longitudinal mode.
However, it is admittedly reported that the Brewster plate causes a large insertion loss because of an imperfection with a profile irregularity of an optically polished plane of the Brewster plate; a very small difference between an optically polished plane of the Brewster plate and the Brewster angle; and surface or internal scattering of the Brewster plate. An etalon also brings about an insertion loss not as great as the Brewster plate due to its surface and internal scattering. The insertion loss induces a drop in an oscillation efficiency of a solid-state laser system.
Moreover, when the Brewster plate and the etalon are incorporated into the cavity resonator, the parts and adjustment of these two optical elements are expensive, which, in turn, adds to the cost of a solid-state laser system.